²²⁶Ra, ²¹⁰Po and lead isotopes in a pit lake water profile in Sweden

A pit lake arises as a consequence of anthropogenic activities in opencast mining areas. These water bodies may be enriched in hazardous stable contaminants and/or in naturally occurring radionuclides depending on the local geological conditions. Mining legacy in Sweden produced hundreds of these pit lakes and most of them are used for recreational purposes in the southern part of the country. In this paper, one pit lake was selected for having enhanced levels of natural radionuclides. Physico-chemical parameters (temperature, pH, oxidation-reduction potential, dissolved oxygen and depth), elemental composition (via Inductive Coupled Plasma Mass Spectrometry) and radiometric characterization (via alpha spectrometry of ²²⁶Ra, ²¹⁰Po and ²¹⁰Pb) were carried along the depth of a 60 m depth pit lake, with the main aim to describe how natural radionuclides and elements behaves with depth in a non-uraniferous pit lake. Based on observed changes in physico-chemical parameters, a thermocline and a chemocline region were identified at around 10 and 30 m depth respectively. Concerning radionuclides, ²²⁶Ra ranged from 75 ± 3 up to 360 ± 12 mBq/kg while ²¹⁰Po ranged from 11 ± 1 up to 71 ± 3 mBq/kg. ²¹⁰Pb distribution with depth was also determined via secular equilibrium with ²¹⁰Po after 2 years and also stable Pb was measured. Disequilibrium ²²⁶Ra-²¹⁰Pb was found and the residence time of ²¹⁰Pb in the water column was assessed. Additionally, different vertical distributions between ²¹⁰Pb and Pb were found which points out different sources for different lead isotopes in the water body.